Titanium Phosphate Glass Microspheres as Microcarriers for In Vitro Bone Cell Tissue Engineering - Biointerfaces: Where Material Meets Biology - page 115

5.3 Visualisation and Quantification of Microsphere
Degradation by Time-lapse Imaging
Before going into the details of how microsphere degradation can be seen and
quantitatively assessed, understanding how glass degradation occurs when
glass specimens are immersed in deionised water is a worthwhile exercise. As
already mentioned, phosphate glasses have the dual capability of being fully
degradable and also easily modifiable to incorporate therapeutic ions, which
means that these glasses are considered prime candidates as bone filling
materials or as scaffolds for bone tissue engineering. The ions released in this
process are mainly degradation products of the P 2 O 5 -Na 2 O-CaO network
skeleton, all of which are non-toxic in nature. 59 Compared to silicate glasses,
weight loss and ion release in phosphate glasses are generally linear and
constant over time. 60 As mentioned previously, the degradation rate can be
altered by several orders of magnitude by engineering the glass structure, 12
thus making this material more suitable for clinical applications. This can be
achieved by stabilising the glass network by doping with several metal oxides.
Furthermore, these ions have been found to induce a biological response
from human cells, thus enhancing the bioactivity of the scaffold material.
The main ions released from phosphate glasses are phosphate groups,
which are known to play a role in enhancing tissue regeneration. The
phosphate groups studied thus far include orthophosphate (PO 4 3 ), linear
species such as pyrophosphate (P 2 O 7 4 ) and tripolyphosphate (P 3 O 10 5 ) and
cyclic species such as cyclic trimetaphosphate (P 3 O 9 3 ). Although higher
phosphate species (P 4 and higher) may be released by these glasses, the
absence of calibration standards containing these species complicates the
identification of these ions by currently available measurement techniques.
These techniques comprise chromatography techniques such as high-per-
formance liquid chromatography and ion chromatography along with other
approaches such as modified end group titration; ion chromatography is
considered the simplest and most effective of these techniques. 61
Phosphate glasses generally degrade and release ions at rates that increase
with time until full glass dissolution. For instance, in ternary glass systems
such as 0.45P 2 O 5 -xCaO-(0.55 x)Na 2 Owherex ¼ 0.30, 0.40 and 0.45 mol.
fraction, all four phosphate species are released in increasing amounts over the
study period. 62 Modifying the content of cationic oxides in the glass is con-
sidered to play a role in determining linear phosphate ion release, and the
presence of single/multiple phases in the glass and the proximity of the glass
composition to a eutectic point in phase diagrams may be contributing factors.
The extent of cross-linking between the branched linear phosphates and the
Ca 21 ions is proportional to the CaO content. 63 The degradation trends ob-
served conform to the model by Bunker et al. 64 which proposes three types of
reactions between the glass and the surrounding water: acid-base reactions,
hydrolysis and hydration. The amount of cyclic P 3 O 9 3 released is generally the
highest among all the investigated phosphate species. From this observation, it
can be inferred that the cyclic species dominates the structure of these glasses.

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